1. Technical Field
The present disclosure relates to a Radio Frequency Identification (RFID) reader, and more particularly, to an RFID reader using a plurality of channels and a Radio Frequency (RF) transmission method thereof.
2. Discussion of Related Art
RFID is an automatic identification method that relies on storing and remotely retrieving data using devices called RFID tags or transponders. An RFID tag is an object that can be applied to or incorporated into a product, animal, or person for the purpose of identification using radio waves. An RFID system may include a plurality of RFID tags (or, transponders), each attached to a different object and having characteristic identification information and an RFID reader for reading the RFID tag information. RFID tags may be classified as a passive type or active type. Passive type RFID tags have no internal power supply. A small amount of electrical current is induced in a passive RFID tag by a radio frequency (RF) signal transmitted from an RFID reader. The induced current provides just enough power in the passive RFID tag to power up and transmit a response to the RFID reader. Active type RFID tags have their own internal power source. The power source is used to power an active type RFID and broadcast a response signal to the RFID reader.
FIG. 1 is a block diagram illustrating a conventional RFID system. Referring to FIG. 1, the RFID system includes an RFID reader 10 and an RFID tag 20. The RFID system transmit signals from the RFID reader 10 and responds to signals from the RFID tag 20, thereby enabling communication between the RFID reader 10 and the RFID tag 20.
The RFID reader 10 includes an antenna 11, an RF filter 12, a directional coupler 13, a transmitter 14, a frequency synthesizer 15, a receiver 16, and a digital signal processor 17. According to a communication protocol of a passive RFID system, the transmitter 14 of the RFID reader 10 alternately transmits a modulated signal and a Continuous Wave (CW) signal in response to a baseband signal received from the digital signal processor 17.
When the RFID reader 10 transmits a modulated signal, the RFID tag 20 only receives the modulated signal, but does not transmit a response signal. Thus, there is no signal received by the RFID reader 10. However, when the RFID reader 10 transmits a CW signal, there is a response signal from the RFID tag 20. Thus, the receiver 16 of the RFID reader 10 needs to process the received signal.
The RFID tag 20 absorbs a part of the CW signal from the RFID reader 10 and reflects the rest of it. The reflected signal is a response signal from the RFID tag 20. The RFID reader 10 receives a signal while transmitting the CW signal. The RFID reader 10 uses the same frequency while transmitting and receiving a signal.
The transmitter 14 generates a CW signal and then transmits it to the directional coupler 13. When the CW signal passes through the directional coupler 13, a part of it is transmitted to the receiver 16, and the rest of it passes through the RF filter 12 and the antenna 11 and then is transmitted to the RFID tag 20. Since the RFID reader 10 uses one antenna for both transmittance and reception, it separates the transmittance from the reception by using the directional coupler 13. For example, a transmit signal may be transmitted only toward the antenna 11 by using the directional coupler 13.
An Ultra-High Frequency (860 MHZ to 960 MHZ) band is used as an RF band of the RFID system. Domestic passive RFID radio equipment is prescribed to use a frequency occupation method (e.g., a Frequency Hopping Spread Spectrum (FHSS) or Listen Before Talk method (LBT)) for access through a channel band width of 200 kHz in a range of 908.5 MHz to 915 MHz.
The FHSS method selects another frequency band to prevent communication interference if a predetermined channel occupation time elapses by using several frequency bands. The FHSS method is commonly employed in the United States due to the availability of a broad frequency band. However, the LBT method is used as standard only when a channel is empty after searching an available channel before transmitting data. The LBT method is commonly employed in Europe due to its narrow frequency band.
An operating environment of the RFID reader 10 is classified according to the number of readers in a predetermined area (e.g., an area within a 1 Km radius). For example, when there are 25 available frequency channels in an operating environment, the environment is considered a single-interrogator environment when only one RFID reader is present therein. The environment is considered a multiple interrogator environment when between 2 and 25 RFID readers are present therein. The environment is considered a dense interrogator environment when more than 25 readers are present therein.
FIG. 2 is a view illustrating a frequency allocation method for conventional domestic RFID/USN radio equipment. As illustrated in FIG. 2, frequency allocation for the domestic passive RFID/USN radio equipment is prescribed to utilize 200 kHz bands ranging from 908.5 MHz to 914 MHz. Accordingly, there are 27 total available frequency bands.
Section <A> of FIG. 2 illustrates a conventional frequency allocation method in a single-interrogator environment or a multiple interrogator environment. A large trapezoid represents a frequency bandwidth when an RFID reader performs a Double-SideBand Amplitude-Shift Keying (DSB-ASK) modulation with respect to a data rate of 40 kbps, and a small trapezoid represents a frequency bandwidth when an RFID tag encodes data through a Frequency Modulation 0 (FM0) method with respect to a data rate of 40 kbps and then responds through a DSB-ASK modulation.
Section <B> of FIG. 2 illustrates a conventional frequency allocation method used in a dense interrogator environment or a multiple interrogator environment. A large trapezoid represents a frequency bandwidth when an RFID reader performs a Single-SideBand Amplitude-Shift Keying (SSB-ASK) or Phase-Reversal Amplitude-Shift Keying (PR-ASK) modulation with respect to a data rate of 40 kbps, and a small trapezoid represents a frequency bandwidth when a tag encodes data through a Miller Subcarrier method with respect to a data rate of 40 kbps and then responds through a DSB-ASK modulation.
When several RFID readers are present an RFID environment, there is a greater possibility of communication interference between the readers. The communication interference can cause performance of RFID readers used in a same area to deteriorate.
Thus, there is a need for an RFID reader that can reduce or prevent interference amongst adjacent RFID readers.